The invention relates to a multi-disc brake, in particular for a commercial road vehicle.
A wet multi-disc brake for a road vehicle is described in DE 10 2008 005 434, which is not a prior publication. DE 10 2008 005 434 proposes to reduce the torque losses which occur in this multi-disc brake during free travel by setting an extra-large release clearance, which is brought back to a normal size again when a braking operation is imminent, even before the actual brake actuation begins.
For this purpose, a brake control system is required that detects events which precede the braking operation, e.g. the release of the gas pedal, and thereupon initiates the reduction in the release clearance.
This method is suitable particularly for electromechanically actuated brakes since the electric motor drive is operated with position control, and hence precise setting of the release clearance in both states is possible. With appropriate adaptation of the position to be set, it is also possible at the same time to provide wear compensation, thus ensuring that no further enlargement of the release clearance occurs.
In the case of a pneumatically actuated brake, which is actuated by use of a brake cylinder supplied with compressed air, precise setting of the enlarged release clearance is very difficult. Although a constant release clearance can be maintained with a conventional slack adjuster, even if wear occurs, situation-dependent setting of different release clearances is not possible, especially by mechanical means.
Enlargement of the release clearance in a manner independent of wear is worthwhile especially in the case of a wet multi-disc clutch since the high viscosity of the oil between the friction surfaces of the plates and the brake discs, i.e. the oil in the release gap, causes frictional losses due to the internal friction in the oil, even when the brake is released, leading to a constantly higher braking resistance and correspondingly higher energy consumption in the operation of the vehicle.
These frictional losses give rise to a permanent residual torque that has to be overcome and is, in particular, dependent on the size of the release clearance.
Although attempts have already been made to enlarge this release clearance in order thereby to allow the oil to flow out of the air gap, the release clearance sizes proposed are unacceptable for higher-speed road vehicles.
Moreover, although wet multi-disc brakes are also subject to wear, this wear increases only up to approximately the magnitude of the enlarged release clearance. That is to say, with a desired enlarged release clearance of about 0.4 mm per friction contact in free travel, it will be necessary to allow for a further 0.4 mm enlargement in the release gap per friction contact owing to wear if this wear is not compensated. If no wear compensation is included, it is important to eliminate this enlarged release clearance in the shortest possible time in the event of a braking operation in order to avoid having to allow for a delayed response from the brake.
It is the underlying object of the invention to develop a multi-disc brake of the type in question in such a way that, by way of a simple design, on the one hand a sufficient enlargement of the release clearance is achieved in free travel to avoid an unwanted residual braking effect and, on the other hand, there is no need to allow for an extended response time during a braking operation.
This object is achieved by a multi-disc brake, in particular for a road vehicle, having plates fixed against rotation, which are arranged parallel to and at a distance from each other and between which in each case one rotatable brake disc is positioned, wherein the radially extending plates and brake discs can be pressed against one another during a braking operation, by use of a brake application device, while overcoming a release clearance and, once the brake is released, the plates can be separated from each other, thereby forming a release clearance. A transmission is provided for transmitting the feed motion during a braking operation. The transmission, on the one hand, is connected in a movement-dependent manner to a pivotable brake lever of the brake application device and, on the other hand, is operatively connected to the plates.
By use of the transmission, which, according to the invention, is arranged between the plates and the brake lever, and which engages on the output side on a ramp ring or on a gearwheel connected to the latter during a braking operation, i.e. when the brake lever is actuated, the rotational speed of the ramp ring is multiplied by a certain factor, preferably by about 5, thus ensuring that the release clearance is traversed quickly.
In this arrangement, a one-way clutch braking device is integrated into the transmission in the region of engagement of the brake lever. The braking device disengages the transmission as soon as an application force takes effect at the brake. At this moment, the actuating force introduced via the brake lever is transmitted directly to a pinion, which meshes with the above-mentioned gearwheel of the ramp ring.
The invention makes it possible to eliminate even a very large release clearance with just a small proportion of the total pivoting travel of the brake lever or a small stroke of a brake cylinder acting on the brake lever and, hence, in a very short time.
The transmission is preferably designed as a planetary transmission. The planet wheels of the transmission are supported in an outer ring, in which an output shaft is guided axially. At one end of the transmission the sun wheel engaging in the planet wheels is held in a manner secure against rotation, while the planet wheels engage on the other side in the internal toothing of a braked annulus.
The action of the transmission will be explained by way of the following example:                Number of brake discs: 3        Number of friction contacts: 6        Release clearance per friction contact: 0.4 mm        Total release clearance: 2.4 mm        Transfer constant of the ramp ring: 4.37° per mm (i.e. the ramp ring must be turned by 4.37° in order to eliminate a release clearance of 1 mm)        Ratio of gearwheel/ramp ring to output shaft: 6.5        
For the total release clearance given, the required turning angle of the ramp ring is 10.48° and that for the output shaft is 68.1°. With a reduction ratio between the brake lever and the output shaft of 5:1, it is necessary to turn the brake lever by 13.6°. Without the use of the transmission, the 68.1° pivoting angle calculated above would be required for the brake lever.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.